Recently, from the environmental standpoint, fuels that do not increase carbon dioxide have been developed. Among these fuels, lower alkyl esters of fatty acids derived from animal and vegetable fats and oils, particularly from vegetable oils (e.g., fatty acid methyl ester) have come into popular use for a fuel in vehicles and the like. Such a fuel is called “biodiesel,” and can be used as is or as a mixed fuel with light oil. The use of biodiesel is rapidly growing.
Fatty acid methyl ester is generally produced by bringing a vegetable oil into contact with methanol in the presence of an acidic catalyst or alkali catalyst at high temperature to esterify the oil, separating layers, and washing with water (Energy & Fuels 1996, 10, 890-895 (ACS Publications), Fuel Processing Technology 2005, 86, 1097-1107 (Elsevier)). The fatty acid methyl ester produced by the conventional method however still contains steryl glucoside which is an impurity originally contained in the raw vegetable oil, even after the step of washing with water. The steryl glucoside is known to cause a serious trouble by depositing as a clog in or around an engine in which the fatty acid methyl ester is introduced (J. Am. Oil. Chem. Soc, 2008, 85, 701-709 (AOCS PRESS), J. Am. Oil. Chem. Soc, 2008, 85, 761-770 (AOCS PRESS)).
For refining a fatty acid alkyl ester, JP-A2008-280252 discloses a method containing thermally oxidizing fatty acid methyl ester, mixing the thermally oxidized fatty acid methyl ester with a nonpolar solvent, leaving the mixture to stand to separate into a lower precipitate and an upper liquid phase, isolating the upper liquid phase, and evaporating the nonpolar solvent from the upper liquid phase.
JP-A07-310090 describes a process of purifying fatty acid methyl ester by adding white clay, while heating, to an oil phase containing fatty acid methyl ester, prepared with given steps, and filtering it to obtain a refined fatty acid methyl ester.
JP-A2009-13268 describes a method for refining a biodiesel fuel such as fatty acid methyl ester, in a method for cleaning a biodiesel fuel by an alkali-catalyzing technique, generating misty water microparticles with a humidifier and allowing to free fall gently in a cleaning tank, and removing alkali ions, glycerol and free fatty acids from the biodiesel fuel while suppressing saponification.
JP-A9-501453 (WO-A95/00609) discloses a method for degumming a fatty substance, bringing the fatty substance into contact with an aqueous solution of a surfactant and/or an emulsifier.